Oracle Database Discussions

Hi
I have two tables,and a I want to set the relationship by FK constraint

-- Table SYSTEM.CLASS

CREATE TABLE "SYSTEM"."CLASS"(
  "CLASS_NAME" Char(40 ),
  "CLASS_ID" Integer CONSTRAINT "SYS_C002720" NOT NULL,
  "STD_ID" Number
)
 

-- Add keys for table SYSTEM.CLASS

ALTER TABLE "SYSTEM"."CLASS" ADD CONSTRAINT "Key3" PRIMARY KEY ("CLASS_ID")
 

-- Table SYSTEM.STUDENT

CREATE TABLE "SYSTEM"."STUDENT"(
  "STD_ID" Number,
  "STD_NAME" Varchar2(25 ) CONSTRAINT "SYS_C002716" NOT NULL,
  "PHONE_NUMBER" Char(20 ),
  "FATHER_ADDRESS" Varchar2(100 ),
  "BASIC_GRADE" Integer,
  "REGISTER_DATE" Date,
  "COMMENT_ON" Char(200 ),
  "STD_ADDRESS" Varchar2(100 ),
  "STUDY_FEE" Float,
  "ITHAD_FEE" Float,
  "TOTAL" Float,
  "STD_RESPONSIBLE" Varchar2(100 ),
  "RESPONSIBLE_RELATION" Varchar2(50 ),
  "MOTHER_NAME" Varchar2(30 ),
  "STUDENT_TYPE" Varchar2(10 )
)
 

-- Add keys for table SYSTEM.STUDENT

ALTER TABLE "SYSTEM"."STUDENT" ADD CONSTRAINT "SYS_C002717" PRIMARY KEY ("STD_ID")
 

the question is:-
whenever I try to add foreign key to table class as a child table
using command:-

alter table class add constraint fk_std_id foreign key(std_id) 
references student(std_id) on delete cascade;
 

I get the error
ERROR at line 1:
ORA-00904: invalid column name
foreign key(std_id)
can someone helps please?